1. Field of the Invention
The invention relates to a manufacturing method of a separator for a fuel cell. More specifically, the invention relates to improving a technique of forming a carbon coating film on a titanium substrate, which has a titanium oxide layer on a surface thereof, by CVD (chemical vapor deposition).
2. Description of Related Art
As a surface treatment technique performed at the time of manufacturing a separator for a fuel cell, a technique of coating a substrate with a carbon-based conductive thin film has been developed which replaces a noble metal plating technique that is a technique of the related art using Au or the like.
As a substrate of a separator, SUS, titanium, or the like is used. Particularly, from the viewpoint of having a high degree of corrosion resistance, a titanium substrate is used. Therefore, a technique of coating a separator substrate made of titanium with a carbon thin film is used with increasing frequency.
In a case in which a carbon thin film is formed on the separator substrate made of titanium by using a raw material of hydrocarbon-based gas by means of plasma CVD (chemical vapor deposition using plasma), it is particularly important to ensure adhesion between the substrate and the carbon thin film. However, it is technically very difficult to ensure the adhesion in many cases. That is, although a titanium oxide layer is spontaneously formed on the uppermost layer of the titanium substrate in the atmosphere, the titanium oxide layer is known as an extremely strong and dense film that exhibits low activity. It is considered that, for this reason, it is difficult to accelerate bonding between the titanium oxide layer and the carbon thin film layer.
Therefore, in order to solve the problem of the adhesion between the titanium oxide layer and the carbon thin film layer, a method of forming an intermediate layer between the titanium oxide layer and the carbon thin film layer is used. Specific examples of such a method include a method of forming an intermediate layer, which is composed of one or more kinds of elements selected from metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W or from semimetallic elements such as Si and B, on a titanium oxide layer (an oxide film as a substrate) and forming a carbon thin film layer on the intermediate layer (for example, see Japanese Patent Application Publication No. 2004-185998 (JP 2004-185998 A)).
Furthermore, as another method for solving the problem of the adhesion between the titanium oxide layer and the carbon thin film layer, there is a method of removing the titanium oxide layer from the surface of a titanium substrate and forming a carbon thin film layer on the surface of the titanium substrate.
However, if the intermediate layer is formed to ensure the adhesion between the oxide film as a substrate and the carbon film, the number of steps for manufacturing the separator is increased, and therefore manufacturing costs are increased. In addition, in the case of removing the titanium oxide layer from the surface of a titanium substrate, the number of the steps is also increased, and therefore the manufacturing costs are increased.